Active implantable medical devices typically include circuits that provide ventricular pacing by delivering stimulation pulses of low energy to different electrodes implanted in the right and/or left ventricle. After the delivery of the stimulation pulse, it is important for the device to collect (detect) the wave of depolarization induced by the stimulation of the ventricle, also called an “evoked wave,” and test for whether the delivered stimulation pulse was effective. This detection, also called the “capture test,” is used in particular to assess the minimum level of stimulation pulse energy needed to cause depolarization of the ventricle, the so-called “pacing threshold.” It is typically necessary to adjust the amplitude and/or width of the stimulation pulse in order to ensure that, first, a stimulation pulse causes an effective evoked wave, i.e., a “capture,” and, second, the delivered energy is not too excessive, so as not to reduce unnecessarily the life of the device due to excessive consumption of energy.
The capture test is also important for monitoring the operation of those devices that deliver cardiac resynchronization therapy (called “CRT”), which are devices equipped with electrodes to stimulate both ventricles. A CRT device can monitor the patient's cardiac rhythm and deliver if necessary pulses to stimulate jointly the left and right ventricles so as to resynchronize them. These respective stimuli are applied with a variable interventricular delay (VVD), which can be positive or negative, and adjusted to resynchronize the contractions of the ventricles and optimize the patient's hemodynamic status. The capture test procedure must ensure that stimulation is effective on the left and the right ventricles because it is an essential condition for their resynchronization. Yet, there are generally different pacing thresholds for the left and right ventricular stimulation sites, which if not accounted for can lead to a defective stimulation. In addition, the device may be confused by detection of an electrostimulated depolarization in the analyzed site, or by a depolarization wave conducted from the nearby site that is indirectly captured, which if not accounted for may result in an erroneous detection of the depolarization waves.
Finally, any change in the interventricular delay may result in a change in the capture test parameters, so it is essential to very accurately follow the operation of the device at each adjustment of the interventricular delay to ensure that the resynchronization therapy is efficient.
There are many techniques known for performing a ventricular capture test. Such techniques are described, for example, in WO 93/02741 A1 and its counterpart U.S. Pat. No. 5,411,533, and in EP 0935979 A1 and its counterpart U.S. Pat. No. 6,181,968 (both assigned to Sorin CRM S.A.S, previously known as ELA Medical), and are based on an analysis of the electrical signals collected by the device.
More recently it has been proposed to test for capture from a signal delivered by a sensor directly detecting the mechanical contraction of the myocardium, which provides immediate information on the response to ventricular pacing, ignoring blanking periods and other limitations inherent in the collection of an electrical signal. In other words, it consists in using a functional signal representative of the cardiac mechanics, instead of a signal due to the electrical propagation of the evoked depolarization wave.
WO 2005/089866 A1 (assigned to Medtronic, Inc.) proposes to use a mechanical contraction sensor in a CRT device for various purposes such as: optimization of the atrioventricular delay (AVD) used for dual chamber pacing, optimization of the interventricular delay (VVD) used for biventricular stimulation for a resynchronization therapy CRT and, alternatively, the discrimination between capture and loss of capture (i.e., when no evoked wave is detected in response to a delivered stimulation pulse) in one or more chambers of the heart during delivery of the stimulation therapy.
U.S. Patent Publication No. 2004/0260351 A1 (Holmstrom et al.) proposes, meanwhile, a detector of an evoked response for a biventricular pacing device, in which a signal of cardiac bioimpedance (or, alternatively, an acceleration signal) is subjected to a morphological analysis to discriminate between effective capture and loss of capture.
For this, the device measures (by a method of least squares applied to successively collected samples over a predetermined time window) the Euclidean distances between, on one hand, the impedance signal collected after each double stimulation and, on the other hand, five reference templates, each corresponding to one of the five following predetermined situations: (i) capture in both ventricles; (ii) capture on the left ventricle only, (iii) capture the right ventricle only, (iv) loss of capture in both ventricles; and (v) intrinsic conduction in the absence of stimulation. The five distances thus determined are compared with respective distance thresholds for classification decisions between capture/loss of capture.
This technique has the disadvantage that the decision regarding the existence and configuration of the captures is taken in reference to a distance, a value that must be relativized and adapted, (from what value do we consider that the signal complies with one of the templates?), the thresholds to which the Euclidean distances are compared being very patient-dependent parameters. Another drawback of this technique is that, as often happens in the biomedical field, the signals compared in pairs (collected signal vs. templates) are not synchronous signals. Thus, if one calculates a distance between two signals shifted by even a small value (e.g., 5 ms), a very large bias may be introduced into the calculation of distance, said bias being present even when the two signals are identical, and thus distort the measurement and the capture test result.
EP 1995685 A2 (Biotronik) describes a method wherein the capture is detected based on predetermined templates and a similarity calculation by distance calculations.
U.S. Patent Publication No. 2008/021336 A1 (assigned to Cardiosync) describes a technique of capture detection based on the extraction of endocardiac acceleration signals for summarizing the characteristic pattern (signature) of the signal, and a detection of the possible capture by means of a unique parameter derived from the pattern.